Gas phase polymerization of monomers, for example, olefin monomers, may be prone to forming “sheets” on the walls of the reactor vessel, particularly on certain catalyst types. Sheeting refers to the adherence of fused catalyst and resin particles to the walls or the dome of a reactor. The sheets vary widely in size. Sheets may be ¼ to ½ inch thick and may be from a few inches to several feet long. They may have a width of 3 inches to more than 18 inches. The sheets may have a core composed of fused polymer, which is oriented in the long direction of the sheets, and their surfaces are covered with granular resin that has fused to the core. The edges of the sheets often have a hairy appearance from strands of fused polymer. Sheeting rapidly plugs product discharge systems and/or disrupts fluidization, leading to the need for costly and time-consuming shutdowns.
Gas phase processes have been found to be particularly prone to sheeting when producing polymers using Ziegler-Natta catalysts, particularly Type III and Type IV Ziegler-Natta catalysts, certain bimodal catalyst systems, and catalyst systems containing metallocene catalyst compounds. While metallocene catalysts yield polymers with unique characteristics, they also present new challenges relative to traditional polymerization systems, in particular, the control of reactor sheeting.
A correlation exists between reactor sheeting and the presence of excess static charges, either positive or negative, in the reactor during polymerization (see, for example, U.S. Pat. Nos. 4,803,251 and 5,391,657). This is evidenced by sudden changes in static levels followed closely by deviation in temperature at the reactor wall. When the static charge levels on the catalyst and resin particles exceed critical levels, electrostatic forces drive the particles to the grounded metal walls of the reactor. The residency of these particles on the reactor wall facilitates melting due to elevated temperatures and particle fusion. Following this, disruption in fluidization patterns is generally evident, such as, for example, catalyst feed interruption, plugging of the product discharge system, and the occurrence of fused agglomerates (sheets) in the product.
It has been found that the presence of polymer coating on the interior surface of the reactor walls, i.e., the surfaces in contact with the polymerization bed, is desirable for reducing the tendency of a gas phase reactor to form sheets. Without being bound by theory, it is believed that the presence of certain reactor wall coatings, for example, a high molecular weight polymer coating, inhibits formation of localized areas of electrostatic charge accumulation on the reactor wall surface. Without being bound by theory, it is further believed that localized areas of electrostatic charge accumulation contribute to the formation of sheets.
Certain chromium compounds aid in the formation of a high molecular weight coating that may be effective in reducing charge buildup on reactor walls and impeding sheet formation. For example, U.S. Pat. Nos. 4,532,311, 4,792,592, and 4,876,320 disclose methods of reducing sheeting in a fluidized bed reactor by introducing a chromium-containing compound into the reactor prior to polymerization and reacting the chromium to form the high molecular weight coating on the walls of the reactor. A particular class of chromium compounds useful in this method is chromocenes, such as bis-cyclopentadienyl chromium.
Two commonly used techniques for treatment or retreatment of reaction systems involve preparation of the wall (for existing reaction systems this required removal of the bad or contaminated polymer coating) and the in situ creation of a new polymer layer. The first of these treatment techniques is a chromocene treatment method (see, for example, U.S. Pat. Nos. 4,532,311, 4,792,592, and 4,876,320). With this method, the walls of the reactor vessel are cleaned, for example, by sandblasting, then the reactor is treated to form a new high molecular weight coating. For existing reactors, the cleaning removes any polymer, including contaminated polymer, from the reactor walls. The reactor is then sealed and purged with nitrogen. A solution of catalyst, for example, chromocene, is injected into the reactor and circulated for an extended period of time. The catalyst deposits on the reactor wall. The deposited catalyst is oxidized and then the reactor is opened for cleaning. After cleaning, ethylene is added to the reactor, and the catalyst reacts with ethylene to form a new polymer coating. After the new polymer coating is formed, the initial product must be monitored closely for the possibility of various quality issues that may arise during initial production due to the presence of sand or concentrated liquid catalyst particles that may remain in the reaction system after treatment. These methods typically use a mixture of the chromium-containing compound in an inert solvent, such as toluene, to contact the reactor walls with the chromium-containing compound. It is generally believed that, the concentration of the chromium-containing compound in the inert solvent is not critical to the process, but is selected so as to assure that the chromium-containing compound is completely dissolved in the solvent. A solution containing about 6 to 8 percent by weight of chromocene in toluene is typically used.
A second treatment method, a retreatment technique, involves hydroblasting the walls of the reactor. In this process, the contaminated polymeric layer is removed with a high-pressure water jet (e.g., hydroblast). The reactor is dried, purged with nitrogen and restarted. The latter restart step employs producing a polymer product at relatively high concentrations of hydrogen so as to produce a high melt index material (i.e., MI≧10) that is less prone to sheeting and readily deposits on the reactor wall to form a new polymer coating. The prior art methods have proven effective to some degree and with some catalyst systems.
The polymer coating formed on the walls of the reactor in contact with the fluidized bed during normal operation (referred to herein as the bed section wall) is typically about 1 to about 10 mils (0.025 to 0.25 millimeters (mm)) in thickness. As used herein, a mil refers to 0.001 inches. Furthermore, the polymer coating is not of uniform thickness throughout the bed section wall. Without being bound by theory, it is believed that the current methods do not provide an even polymer coating on the bed section wall because the chromium is not deposited evenly on the bed section wall. Still further, it is also believed that a significant amount of the deposited chromium may be deactivated before it is reacted with a monomer to form the polymer coating. It is theorized that the relatively thin and uneven nature of the polymer coatings of the current technology may limit the effectiveness of the polymer coating at preventing sheeting, and may shorten the effective life of the polymer coating.
Furthermore, with the current methods, it may also be necessary to operate the polymerization system on catalyst systems that are less prone to sheeting, for example, non-metallocene catalyst systems. This may result in product supply issues.
Still further, it has been found that when using prior art methods to treat a reactor, the reactor must be periodically re-treated, as the polymer coating degrades over time. The time between treatments varies depending on the effectiveness of the last treatment, products produced, catalyst type, and a number of other factors.
Referring to Prior Art FIG. 1, depositing chromium on the interior surfaces of the polymerization reactor is typically done by the injection of a chromocene containing solution through one of the catalyst injection points 2 of the reactor 4. The solution may be injected through a single straight tube, or may be injected through a single tube with a spray nozzle located at the end of the tube. An inert, such as nitrogen, is circulated through the reactor 4 by the cycle compressor 6 while the solution is slowly injected over a period of time, at least one to three hours, and may be as long as eight hours. The reaction system then circulates the mixture for a period of about twenty hours. In this method, it has been found that the level of chromium deposited on the bed section wall is significantly lower than the level of chromium deposits in the bottom head and on the bottom of the plate. Thus, the method preferentially deposits the chromium on the distributor plate 10 and in various parts of the reactor system other than the bed section wall, such as the cycle compressor 6, and the cycle cooler 12. The chromium deposited on the distributor plate 10 and other parts of the reaction system by the prior art method typically must be cleaned off before reacting the chromium to form the polymer coating.
Because the excess chromium is cleaned from the various parts of the reaction system before forming the coating, the reaction system is opened to the air before the coating is reacted with a monomer. Before opening and cleaning the reactor, the chromium is oxidized by exposure to a relatively low level of oxygen (e.g., about 100 ppmv). Without being bound by theory, it is believed that further oxidation of the chromium occurs when the reaction system is opened to air. Still further, it is theorized that the exposure to air results in excess oxidation of the chromium and a lower chromium activity when forming the polymer coating.
Other background references include U.S. Pat. Nos. 3,449,062, 3,539,293, 4,460,330, 6,335,402; U.S. Patent Application Publication No. 2002/026018; and WO 1997/49771, WO 2004/029098.
It is thus desirable to develop an improvement to a method for polymerization of alpha-olefins in the presence of a catalyst prone to sheeting. It is also desirable to provide a method of treating a gas phase fluidized bed polymerization reactor to preferentially deposit a chromium containing compound on the bed section walls of the reactor, and form a high molecular weight coating on the bed section walls of the reactor that is thicker, and more uniform. It is also desirable to provide an apparatus to effectively deliver a treatment to the bed section walls of a gas phase fluidized bed polymerization reactor.